1. Field of the Invention
This invention relates to a sizing device useful in coal washing and mineral extraction and more particularly, to a sieve bend for separating particles less than a predetermined dimension from a slurry.
2. Description of the Prior Art
In commercial coal-cleaning plants, sieve bends are used to remove either -0.15 mm pyrite (containing sulfur) or clay (containing high ash), or both. Pyrite and clay contaminant clean-coal, potentially causing problems in a utility boiler.
Although fine-aperture sieves have high capacities and sizing efficiencies when first installed, their performance deteriorates over time because coarse particles, 6.3 mm (1/4in) or greater in size, errantly enter the fine-coal circuit feeding the sieves and obstruct the feed throat to the sieve. Variations in flow of slurry cause the headbox feeding the sieve to empty and reduce the velocity of the slurry feeding over the sieve. The sieve bend capacity is dependent upon three variables: the velocity of the slurry flowing over the sieve, the thickness of the layer of slurry that is diverted through the slotted openings on the sieve, and the number of wires the slurry contacts while traveling over the sieve. Further, the profile of each wire dulls during abrasive wear and use, reducing the actual thickness of the layer of slurry each wire diverts through the slot. Further, the slurry misses a portion of the sieve's surface because of misalignment between the sieve and the headbox feeding the sieve.
In U.S. Pat. No. 2,748,941 granted June 5, 1956 to F. J. Fontein et al., an apparatus whose screening deck can be reversed in a simple manner is shown. The screen separates liquid suspended particles.
In U.S. Pat. No. 2,814,388 granted on November 26, 1957 to F. J. Fontein et al., a screening device is shown in which the material to be screened, consisting of a mixture of solid particles of different grain size suspended in the liquid, is fed tangentially as a thin layer to the concave side of a screen deck which consists of a cylindrically bent screening surface.
In U.S. Pat. No. 2,916,142 granted on December 8, 1959 to F. J. Fontein, shows in FIG. 8, a plate 124 and outer wall 125 which operates to deliver the mixture along a tangential path to a curved screening deck 126 composed of bars 127.
In U.S. Pat. No. 3,007,574 granted on November 7, 1961 to J. de. Koning, in FIG. 1 a slot-shaped aperture 3 is shown mounted over sieve bend 4 including a screening deck 5 comprising horizontal bars 6. Shaft 20 and shaft 17 are provided to facilitate reversing screening deck 5 end for end.
In U.S. Pat. No. 3,353,674 granted November 21, 1967 to J. N. J. Leeman, a sieve bend 4 is shown having a spout 2 communicating with a throat 3 wherein the throat 3 is defined by a sloping wall 5 tangentially leading into the upper end of the curved screening deck 6 of the sieve bend 4 and a movable wall comprising a resilient rubber plate 7 in such a way that a wall 5 and plate 7 converge downwardly towards the sieve bend 4. Rubber plate 7 may be hinged and is deflectable, to prevent obstruction of the feeding throat by coarse particles contained in the material supplied.
In U.S. Pat. No. 3,446,349 granted on May 27, 1969 to W. Benzon, a sieve bend is described with means for depositing slurry onto the sieve bend across the entire width thereof under a static head at a velocity sufficient to prevent build-up of solid particles at the discharge end of the sieve bend. A means for intermittently rapping the sieve bend is further described.
In U.S. Pat. No. 4,113,626 granted September 12, 1978 to T. E. Detcher, a screen assembly having a semi-parabolic concave vertically extending surface defined by horizontally extending wedge shaped bars fixed in spaced parallel relation to approximately curved support means is described.